This can be answered by asking why do we all have different skin colours: Melanin pigmentation. Years of evolution made different people around the worrld different skin colours as an protective mechanism against high levels of UVR exposure, the closer people were to the eqautor, the hotter it would be, so many people living in tropical and desert like climates had darker skin. To answer about folate though, people are darker skin tones living near the equator had dark skin to prevent folate deficiency. So yes everyone has folate, but some people are darker skin to better protect it and their skin.
Hope this helps, HAVE A BLESSED AND WONDERFUL DAY! As well as a great Superbowl Weekend! :-)
